LONDON -- Britain's bumblebee population is under threat in a crisis that could wipe out entire species and have a devastating knock-on effect on agriculture, scientists say.

The furry yellow-and-black creatures, essential for pollination, are being killed off by pesticides and agricultural intensification, which have cut back on hedgerows and removed their source of food.

"There just aren't enough flowers around," Professor Dave Goulson, the director of the Bumblebee Conservation Trust at the University of Stirling in Scotland, said on Monday.

"If we knock out an important group like bumblebees, it can have a huge knock-on impact on other things, such as the pollination of important crops and flowers."

Britain once had around 25 native species of bumblebee, but three of those have been wiped out in the past 50 years and 10 more are now "severely threatened", Goulson said.

"There are two that are teetering on the edge of extinction and could be gone in five to 10 years quite easily," he said.

The loss of species could lead to sweeping changes in Britain's countryside, with many rare plants disappearing and the production of crops such as raspberries, oil-seed rape, runner beans and broad beans sharply curtailed.

Some estimates say bees carry out pollination worth around $400 million to Britain's farmers each year.

"If you make one thing rarer -- like bumblebees -- it feeds back to other things, which quickly become rarer too," Goulson said.

The problem is not just in Britain -- it has been spotted in North America and Europe too. That could make it more difficult to reintroduce any species that goes extinct locally.

Goulson and other scientists want farmers to adopt more wildlife-friendly farming methods to help sustain the bumblebee population, and are encouraging people to look after the bees.

"They're a quintessential part of the summer -- the buzz of bumblebees on the flowers in the garden," said Goulson.